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ALL ABOARD THE CLEAN ENGINE

Automotive Emissions and Onboard Diagnostics

by John West & Mark Stachew

Start ý Why Onboard Diagnostics? ý Major Components ý Catalytic Converter ý Oxygen Sensor ý EGR ý Fuel System Adaptive Learning ý Misfire Detection ý The Need for Speed ý New Regulations ý 0 to 100 KB in 6 Seconds ý Sources and PDF

OXYGEN SENSOR

When the air/fuel mixture is rich and there is little oxygen in the exhaust, the difference in oxygen levels across the sensing element generates a voltage through the platinum electrodes of the oxygen sensor (typically 0.8 to 0.9 V). When the air/fuel mixture is lean and there is more oxygen in the exhaust, the voltage of the sensor drops down to 0.1 to 0.3 V. When the air/fuel mixture is perfectly balanced and combustion is ideal, the sensorýs output voltage is around 0.45 V.

The voltage signal of the oxygen sensor is monitored by the engine management system (EMS) to regulate the fuel mixture (see Figure 2). When the EMS sees a rich signal, it commands the fuel mixture to go lean. And in turn, when the EMS sees a lean signal, it commands the fuel mixture to go rich. Cycling back and forth from rich to lean averages out the overall air/fuel mixture, minimizing emissions and helping the catalytic converter operate at peak efficiency.

Figure 2ýA typical oxygen sensor cross section can be seen here.

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